An Appeal to motive is a form of logical fallacy that's related to ad hominem attacks. This particular fallacy is when an argument one is making is questioned due to his or her supposed motives.

The problem with this sort of fallacy is that addressing one's motives, instead of the argument put forth, is a type of red herring. Additionally, unless the person appealing to motive is telepathic, how could the person know one's secret motives, and whether they are part of the decision making process?